There are a multitude of patents dealing with the measurement of refractive indices. The vast majority of those patents deal solely with measurements on liquids. The remaining patents for devices that measure refractive indices are roughly broken into two categories: reflectometers and refractometers. Reflectometers use light reflection to make measurements and refractometers rely on the refraction of light to make measurements.
The following patents describe refractometer devices capable of measuring the refractive index of solid samples. U.S. Pat. Nos. 2,383,347, 4,692,024, 5,572,314, and 7,199,871. The following patents describe reflectometer devices capable of measuring the refractive index of solid samples, U.S. Pat. Nos. 3,402,631, 3,751,162, and 4,899,055. The following patents do not really fall into the exclusive categories of refractometer or reflectometer, yet are still devices capable of measuring the refractive index of solid samples, 2004/0233434 A1, U.S. Pat. Nos. 7,292,389, 6,037,579, 6,937,333, and 6,924,893.
U.S. Pat. No. 6,181,427 “Compact Optical Reflectometer System” describes a device for collecting reflectance data and images from a sample. The optical path includes a spectrometer for measurement of properties other than refractive index. It is to be used with thin films, not crystalline samples. U.S. Patent Ser. No. 2003/0086083 A1, “Optical Metrology tool with dual camera path for simultaneous high and low magnification imaging” describes a device for imaging the surface of a sample at two levels of magnification. This particular device lacks the ability to make reflectance measurements, which can be used to calculate refractive indices.
U.S. Pat. Nos. 7,099,081, 7,248,362, and Patent Application US2006/0197948AI “Small-Spot spectrometry instrument with reduced polarization and multiple-element depolarizer therefore” describes a device which contains illumination and collection optics. This instrument is primarily designed to look at thin films or wafers of material, not crystalline samples.
The following patents describe light scattering devices operating at scattering angles of 90 degrees and/or 180 degrees: U.S. Pat. Nos. 6,643,012, 6,025,918, US 2004/0145735, U.S. Pat. Nos. 5,661,557, 4,575,629, 5,373,358, 5,684,580, 5,088,820, 4,586,819, 4,505,586, 6,850,323, 3,469,920, 4,080,073, 5,870,188, 6,307,625, 5,999,255, 6,205,354, 5,949,532, 7,039,452, 6,868,285, 6,281,971, 5,607,643, 5,869,346, 5,753,449, 6,667,070, 5,621,523, 6,545,755, 4,648,714, Re. 34,153, U.S. Pat. Nos. 4,008,961, 6,795,177, 6,747,735, 5,991,653, 5,841,139, 5,351,250, 4,269,509, 4,197,009, 4,270,864, 6,897,951, 4,620,284, 3,704,955, 5,751,289, 4,068,953, 4,127,329, 4,856,897, 5,247,343, 5,786,893, 2,940,355, 3,807,862, 5,377,004, 6,040,906, 3,516,744, 3,542,481, 6,614,523, 3,414,354, 4,798,463, 5,442,438, 5,510,894, 4,081,215, 5,255,067, 6,352,502, US 2005/0002028, U.S. Pat. No. 5,110,204. These patents deal exclusively with light scattering phenomenon. Several of these devices employ fiber optics as part of the device, but it is usually a small contribution.
The following patents describe reflective devices where the incidence and reflective angles can be varied from approximately 0 degrees to approximately 90 degrees: U.S. Pat. Nos. 6,897,955, 4,647,207, 6,734,967, 5,608,526, 6,859,278, 4,790,659, 7,023,549, 5,166,752, 5,416,588, 6,590,656, 6,483,580, 6,753,961, 4,834,539, 5,963,327, and 4,585,348. These patents are primarily concerned with ellipsometers and other reflection-type devices, which is a much different phenomenon than light scattering. Several of the devices employ a small amount of fiber optics.
The following patents describe fiber optic devices applied to light scattering: U.S. Pat. Nos. 5,615,673, 6,486,948, 5,521,703, 6,373,567, 6,333,784, 5,911,017, 6,028,666, 6,006,001, 4,630,923, 5,765,948, 3,770,350, 6,061,134, 6,115,528, 5,751,415, 6,100,975, 5,184,521, and 5,783,389. These patents use fiber optics extensively in the light scattering process. However, these devices still gather light at scatterings angle of 90 degrees or 180 degrees.
U.S. Pat. No. 5,262,644; “Remote spectroscopy for Raman and Brillouin Scattering” describes a device for collecting Raman and Brillouin spectra remotely, using optical fibers. The sample being investigated is held in a container and the optical fibers positioned around it. The apparatus does allow the fibers to be placed at a multitude of angles, fixed into position by a ring collar. The incident fiber carries infrared light from a suitable laser source and the collection fiber takes the scattered light to various instruments and detectors. Nothing in the apparatus allows control over the polarization of the incident or scattered light. In addition, the apparatus does not appear to allow the sample container to be independently moved and aligned, relative to the fiber optic cables.
The apparatus of another device was published in the journal Review of Scientific Instrumentation 60(8) in August 1989 in an article entitled, “Light scattering apparatus for angular dependent studies on anisotropic materials.” This device also employs a four circle apparatus with respect to the sample, but was entirely constructed of conventional mirrors and optics. The light must be steered under the rotation stage and then directed exactly perpendicular to the table surface, through the precise center of rotation of the stage. Failure to achieve this level of precision results in a beam that walks or is clipped when the rotation arm is moved from side to side, when all the incident optical components are in place. In addition, the light is directed under the arm from the center of the stage before being reflected upward and through the focusing optics toward the sample. Every mirror reflection represents a loss of light intensity, a possible change in light polarization, and an opto-mechanical mount that has to be adjusted and will eventually migrate out of alignment. The result works, but is very difficult to align and maintain. In particular, the mirror mounts tended to drift over time, meaning that the device would have to be completely re-aligned in order to be functional again. The use of so many mirrors also limited the range of motion of the rotating arm. Because the light had to be directed under the rotation stage, the mount for the sample had to be built off-center and contain multiple supports to properly distribute the weight. These supports severely limited the range of motion for the rotation arm assembly. This limitation means that the final setup did not have the capability to do a true 180 degree back scatter experiment. Finally, the placement of the pick-off beam to align the sample on an axis orthogonal to the rotation arm had the potential to send light directly down the collection optics path into the detector, causing catastrophic damage.
All of the shortcomings of the devices described above are resolved in the invention described below. An ideal light scattering apparatus and method would include a light that is steered using fiber optics, eliminating the problems associated with multiple mirrors and their mounts. The invention described below is simpler which makes it much easier to align and is also extremely stable over time, and complete re-alignments are no longer necessary. In addition, the sample is now aligned using the scattering optical axis, which means a true 180 degrees backscatter experiment is now possible, and there is very little possibility of sending intense light into the detector. The range of the rotation arm is also greatly increased, making it possible to probe all possible scattering geometries. The light polarization is always maintained and the beam does not walk and is not clipped when the rotation arm is moved. The invention described below is more flexible and adaptable than any of the prior devices.
It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not to be viewed as being restrictive of the invention, as claimed. Further advantages of this invention will be apparent after a review of the following detailed description of the disclosed embodiments, which are illustrated schematically in the accompanying drawings and in the appended claims.